Lunar L1500

[Dr Zeus]

Product 3



Got this little amp in for repair. The owner said it may be made in Louisianna. The board is stamped, "Designed and drawn by K.Lanclos Q.A.S. Opelousas, LA. There are also some fun things written on the board, like "Secret Circuit Under Caps" and, "What Does This Do" under the 50v rail caps.





The amp uses 6x IRF3205 in the PS, and 45N20B outputs. When powered, sometimes the amp will will produce +-44vDC rail voltage. If let for sitting, eventually the amp will just go into protection (red LED) and the rail voltage drops to practically nothing. There is nothing visually out of wack other than a 1/2w 6.8k good reading but darkened resistor pictures near the red LED which likes to run hot. The voltage across this resistor while the amp is powered and NOT in protection is ~86vDC.

While the amp is powered and not in protection there is observed AC signal output proportionate to what is inputted from my frequensy generator; however the signal is very very weak, dirty audio, and only slightly more amplified at full gain than measuring my frequency generator directly with the o-scope.

[Dr Zeus]

I found out that 1/2 burnt resistor is mainly just going to the amp's primary power LED. I'm sure it is just being used to supply the proper voltage off the rail to the power LED.

[ppia600]

Yeah those amps were made in Louisiana, I remember some guy bragging about his two channel. It was pretty small and had the hammered metal finish. Don't know if they were worth anything, never heard one play.

[ian33]

remove the outputs and let it idle.one might be leaking

[Dr Zeus]

I figured out why the amp sometimes goes into protect; its like it senses my meter probes and freaks out.

I do think I found a problem with either the primary opAmp or something around it. Unless this opAmp is being used as something else, the outputs are not totally up to par. Referencing the amp's CT, the LF347N is getting +-15.2vDC at power pins. Two of the four outputs are measuring 14.4v. A third output is measuring 0.250vDC, and the last is measuring -0.060vDC. I think I want to at least pull this opAmp out this to check out what reads then.

http://focus.ti.com/lit/ds/symlink/lf347b.pdf

I have a few spare quad opAmps; LM324N and LM837N. Worth testing with?

[Perry Babin]

Post the voltage on the pins.

Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
Pin 9:
Pin 10:
Pin 11:
Pin 12:
Pin 13:
Pin 14:

[Dr Zeus]

LF347N quad opamp

Pin 1: -0.274
Pin 2: -0.014
Pin 3: 0.000
Pin 4: 15.45
Pin 5: -0.274
Pin 6: -0.076
Pin 7: -0.067
Pin 8: 14.88
Pin 9: 14.88
Pin 10: 14.34
Pin 11: -15.54
Pin 12: -0.048
Pin 13: 15.40
Pin 14: 15.40

[Perry Babin]

The section with pins 12, 13 and 14 is defective unless something is driving DC back into the output of the op-amp.

[Dr Zeus]

I couldnt find much sense in why pins 14&15 had ~15v on them (other than because they were tied directly together) so on a hunch I clipped the pins off the side of the opAmp which detached them from the PCB. Now I read the following:

LF347N quad opamp

Pin 1: -0.474
Pin 2: -0.027
Pin 3: 0.000
Pin 4: 15.45
Pin 5: -0.474 (tied to Pin 1)
Pin 6: -0.114
Pin 7: -0.100
Pin 8: -14 *See below
Pin 9: -14 *See below
Pin 10: -0.003
Pin 11: -15.54
Pin 12: -0.048
Pad 13: -0.003
Pad 14: -0.003

Pin 8&9 are connected. When I first touch my meter to them, I read ~-0.3v, but after ~4 seconds the voltage drops to -14v. If I release and re-apply meter, it demonstrates the behavior again. This was not the case before clipping Pin 13&14.

I went ahead and also clipped Pin 12 and the above made no difference.

Im tempted now to clip Pins 8,9,10...

[Dr Zeus]

OK Looks like the amp really did just have a bad opAmp. I still need to replace it, but I am now able to get some full volume audio out of this amp (With pins 12,13,14 clipped) and a test speaker attached. It has a wicked whining coming through but at least I've made progress.